DNA viruses are responsible for extensive morbidity and mortality on a worldwide basis. Viral homologs of anti-apoptotic Bcl-2 family proteins are encoded in the genomes of several classes of DNA viruses. In Vaccinia Virus (VV), a poxvirus-family member that has served as a paradigm for investigations of many aspects of host-pathogen interactions, at least two viral Bcl-2 genes have been identified, F1L and N1L. Neither F1L nor N1L is required for VV infection or replication, but both of these genes make strong contributions to virulence in vivo. Thus, these viral Bcl-2 (vBcl-2) homologs are critically important for in vivo viral pathogenicity of VV. While the anti-apoptotic activity of F1L and N1L is an obvious candidate for explaining their contribution to viral virulence, we have discovered that F1L and N1L have additional functions that include binding to and suppressing the pro-inflammatory actions of NLR-family proteins, important mediators of innate immunity. Viral Bcl-2 proteins are also known to bind Beclin and suppress autophagy, recently recognized as a host defense mechanism against pathogens. We hypothesize the vBcl-2 homologs are multifunctional proteins that utilize 3 discrete mechanisms to thwart host defense mechanisms: (a) suppression of apoptosis; (b) inhibition of autophagy; and (c) interference with NLR-mediated innate immune responses. The hypothesis that we will test is that neutralization by vBcl-2 proteins of each of these 3 classes of host cell targets significantly contributes to virulence. Specifically, we will: (1) Produce site-specific mutations in F1L and N1L that selectively abolish their ability to interact with (a) pro-apoptotic Bcl-2 family proteins [apoptosis]; (b) NLRs [inflammation]; and (c) Beclin [autophagy]; (2) Test the effects of the engineered vBcl-2 proteins on apoptosis, inflammation, and autophagy in cultured cells; (3) Produce recombinant vaccinia viruses with knock-in of F1L and N1L mutants; and (4) Compare the virulence of these recombinant vaccinia viruses in mice. By using VV as a model system, our results will lay a foundation for understanding the role of viral Bcl-2 homologs in viral pathogenicity, thus serving as a paradigm for other DNA viruses that contain vBcl-2 genes and that cause debilitating human diseases. Also, by learning how viruses interfere with apoptosis, autophagy, and inflammation, the information generated may reveal novel strategies for mimicking aspects of vBcl-2 function in therapeutically useful ways for addressing disorders in which excessive apoptosis, autophagy, and inflammation play central roles.